1. Field of the Invention
The present invention relates to a developing apparatus, an image forming apparatus equipped with the developing apparatus, and developing method of those apparatuses, wherein a latent image on a photosensitive body gets developed with toner. More particularly, it relates to a developing apparatus, an image forming apparatus equipped with the developing apparatus, and developing method of those apparatuses, wherein development is made in accordance with contact developing with one-component non-magnetic toner.
2. Description of Related Art
Various proposals have conventionally been made with respect to developing apparatuses for image forming apparatuses (see JP laid-open Patent Publication No. 2003-29507, JP Patent No. 3363593, and the like). Development methods for developing apparatuses are classified into two, namely, one-component developing and two-component developing. Developer without carrier is used for one-component developing, whereas developer including toner and carrier is used for two-component developing. Since one-component developing does not need mechanism for agitating developer, it is convenient for miniaturization of mechanical structure in comparison with two-component developing. A developing apparatus of one-component developing is equipped with a regulating member for regulating thickness of a toner layer on a toner carrier.
one-component developing is further classified into two types depending on developing voltage application, namely, DC developing in which developing voltage is limited to direct current components, and AC developing in which AC bias components are superimposed on developing voltage. Furthermore, one-component developing is classified further depending on contact state of a facing portion with respect to a toner carrier and an image carrier (so-called, development region). That is, depending on contact state of the facing portion, it is classified into non-contact developing in which a predetermined space is provided between a toner carrier and an image carrier, and contact developing in which both of the carriers are in contact with each other.
Non-contact developing is excellent in reproducing of dots and fine lines because edge effect can be obtained due to space of development regions. On the other hand, as the space fluctuates, developing electrical field also fluctuates, which causes noises such as unevenness of an image and the like. This is a drawback of non-contact developing. Therefore, strict control of space accuracy is required for non-contact developing, which overloads control system. Especially, a combination of non-contact developing and AC developing overloads power system. This is because high-amplitude development bias (1 kV or higher, at peak-to-peak) is required.
On the other hand, contact developing requires comparatively less development bias. Therefore, contact developing is advantageous in terms of power load. Not to mention, contact developing does not require space control, which is another advantageous point. However, edge effect cannot be expected with contact developing. Therefore, contact developing is disadvantageous for reproducibility of dots and fine lines compared with non-contact developing. Particularly, under recent demand that resolution of about 600–1200 dpi has been required for higher image quality, contact developing faces difficulties in accurate reproduction of a highlight section and sufficient gradation degree.
Therefore, there is devised a combination of contact developing and AC developing for compromising between load on an apparatus and image quality. By doing so, it is conceived that edge effect can work for improving reproducibility of dots and fine lines despite contact developing. This is because such a method combination can make toner fly at non-contact regions upstream and downstream of a development region.
However, there have actually been difficulties in combining contact developing and AC developing. For example, a developing apparatus of JP laid-open Patent Publication No. 2003-29507 tends to create unevenness of image density when AC bias is set high. Furthermore, application of contact developing and one-component developing to this developing apparatus causes current leakage between its image carrier and development member, which results in fog on a background section of an image. In this developing apparatus, an absolute value of voltage difference between peak voltage V1 for development bias and imaging section voltage VL for an image carrier is set to 600 V or higher and this absolute value is considered to be too large for contact developing. Furthermore, application of one-component developing to a developing apparatus of JP Patent No. 3363593 results in poor reproducibility of a highlight section in an image due to the above-described reason, which is problematic.